HS2's tunnelling team are celebrating completing half of the 8.4-mile twin bored Northolt Tunnel underneath London.

Four tunnel boring machines (TBM) are being used the complete the tunnel that will carry high-speed trains between HS2's super-hub station at Old Oak Common, west London, and the outskirts of the capital at West Ruislip.

The machines will dig out 16.8 miles of tunnel overall making it the second longest tunnel on the new railway after one recently excavated beneath the Chiltern Hills.

The first machine, named Sushila after a local teacher, was launched eastwards from West Ruislip in October 2022.

This was followed one month later by TBM Caroline, named after astrologer Caroline Herschel, which is used to excavate the adjacent tunnel.

Both will complete five miles of the route, finishing their journeys at the Green Park Way vent shaft in Greenford where they will be lifted out. 

HS2's Northolt Tunnel being bored by TBM Anne from the Victoria Road Crossover Box (credit: HS2).

In February this year, TBM Emily, which is named after Emily Sophia Taylor, began its journey west from HS2's Victoria Road Crossover Box site near Old Oak Common towards Greenford.

She was joined by the fourth machine, named Anne, after Lady Anne Byron, in May. They will both complete 3.4 miles of the tunnel.

Combined, the four machines have now travelled over 8.4 miles – half of the total distance for the twin-bored tunnel.

The completion of each tunnel drive will be staggered as they reach the final destination at Green Park Way, with the first TBM, Sushila, arriving later this year.

Each machine operates like an underground factory, excavating the tunnels, lining them with pre-cast concrete tunnel segments, grouting them into place before moving forward at an average speed of 16 metres per day.

Teams work around the clock below ground on the TBMs along with teams on the surface supporting them. 

HS2 London Tunnel Map Infographic (credit: HS2).

Marking the significance of the tunnelling milestone, Malcolm Codling, Client Director for HS2 Ltd, said: "Our London tunnels team are making significant progress taking HS2 from design to reality underneath the capital, building Britain's new transformational low carbon railway.

"Completing half of the Northolt Tunnel has come after years of work from the team and we are now looking forwards to the TBMs reaching the end of their journeys, with the first arriving at Green Park Way in Greenford later this year."

As the TBMs progress, other vital features of the tunnel are put in place such as cross passages, vent shafts and headhouses.

Along the Northolt Tunnel there are five shafts and headhouses which will provide ventilation systems and emergency access to the tunnel when in operation.

The excavated material is being put to beneficial reuse. Around the West Ruislip portal, it is being used around the Copthall Green Tunnel, to remodel a nearby golf course that has been impacted by HS2's tunnelling and is being used sustainably to create new green spaces in the Hillingdon area surrounding the new railway.

London Clay removed from the Victoria Road Crossover Box side of the tunnel, is being taken to the London Logistics hub using a spoil conveyor.

From there it is taken by rail to beneficial reuse sites across the UK in Rugby, Kent and Cambridgeshire.

Drone shot of the HS2 Euston Tunnel site (credit: HS2).

The logistics hub has now transported over 2 million tonnes of materials to these sites, which are being used to backfill quarries to support housing developments and build wildlife areas for birds.

The Northolt Tunnel is being built by a joint venture formed of the companies Skanska, Costain and STRABAG (SCS). The same JV will also construct the Euston Tunnel eastward from Old Oak Common to the centre of London.

James Richardson, Managing Director of Skanska Costain STRABAG joint venture, said: "Reaching the halfway point of our tunnelling programme for the Northolt Tunnel demonstrates the huge progress achieved to date on one of the most complex and ambitious programmes embarked upon in the UK.

"Alongside the tunnelling operations we are now moving into the fitout stage of the works in readiness for track and rail systems to be installed.

"At Old Oak Common, we are gearing up to assemble and install our final two TBMs, ready to take the high-speed line into central London."

Preparations for the final tunnel between Old Oak Common station and Euston are underway, with two TBMs arriving this summer.

Timings and funding for the Euston Tunnel are being confirmed with the government.

Once complete, HS2 will enhance connections between London and the West Midlands, extending further north on existing lines.

HST say the new railway will offer quicker, more reliable journeys, boost economic growth, and free up space for local trains on the congested West Coast Main Line.

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